Proximity sensor, especially for ignition of the warhead of a shell directed against an aprroaching missile

ABSTRACT

A proximity sensor ( 11 ) is intended to initiate the warhead of a defence shell which is fired against an attacking projectile from the object to be protected in order to interfere with or even destroy the functionality of the attacker. For that purpose the proximity sensor ( 11 ) should not yet respond to the attacker which is to be defended against by the shell appearing ahead in the direction of flight thereof; rather, the proximity sensor is to respond only when the attacker to be defended against is detected ahead inclinedly at an operatively optimal distance. Such a response characteristic in the form of the wall of a hollow cone is afforded if an annular detector element ( 12 ) is arranged in the image focus plane behind a positive cylindrical lens ( 14 ).

[0001] The invention concerns a proximity sensor as set forth in the classifying portion of claim 1.

[0002] Such a proximity sensor is required for an active defence system in which the object under attack fires against the attacking projectile, in accordance with U.S. Pat. No. 5,661,254A a defence shell with a fragmentation warhead or, in accordance with U.S. Pat. No. 6,244,156B1, a defence shell with a blast warhead. The attacker may be a projectile which is fired without a drive (shell) or a projectile fitted with its own drive (missile). The defence is implemented by damage to or triggering of the firing sensor system of the attacker or by influencing the attack trajectory thereof. Such a scenario is diagrammatically illustrated for a case by way of example in DE 196 01 756 C1. As shown therein it cannot realistically be assumed that a collision will take place between the attacker and the defence shell. Therefore the warhead of the defence shell must be fired immediately prior to its flying past, because that then affords the optimum combat situation relative to the attacker, for the fragmentation or blast action of the defence shell.

[0003] Radar fuses are used for distance triggering in relation to aerial targets. Their lobe-shaped sensitivity characteristic however does not afford clear and reproducible firing information, in the illustrated passing situation. A particular disadvantage with radar fuses is that they also and even preferably lead to target contact in a direction coaxially ahead, and thus in a proximity situation, which is extremely disadvantageous in terms of the distance involved and for the radially oriented action of the defence warhead.

[0004] In consideration of those factors the object of the present invention is to provide a proximity sensor for operatively optimised triggering of the warhead of a defence shell.

[0005] According to the invention that object is attained by the essential features recited in claim 1. In accordance therewith the reception characteristic of the proximity sensor according to the invention describes the wall of a hollow cone which is coaxial with respect to the system axis of the defence shell and which opens ahead in the direction of flight, towards the approaching attacker. The proximity sensor responds, that is to say results in initiation of the defence warhead, when the attacker to be defended against passes the wall of the hollow cone. By virtue of the cone geometry, that event is correspondingly further in front of the defence shell, the greater the respective radial distance from the axis of the system and thus from the trajectory of the defence shell. As a result the effect of the warhead (a fragmentation cone which is propagated radially therearound or a corresponding pressure wave) has a correspondingly longer time to be propagated inter alia in a direction towards the approaching attacker, the further away it is when fuse triggering takes place. In contrast an approximately coaxial proximity situation is not detected by the sensor because that takes place in the insensitive interior of the hollow cone. As long as the attacker to be defended against is approaching in the interior of the hollow cone configuration of the sensor characteristic therefore, the sensor does not yet respond, as is desired, because the effect of the defence warhead is not oriented axially ahead but radially with respect to the defence trajectory; therefore, fuse triggering is delayed until a lateral flypast situation is impending.

[0006] Preferably the proximity sensor is operated optoelectronically in the infrared range, for detecting heated tips and edges of the attacker; more specifically by means of a cylinder of thermooptically conducting material, the mutually opposite end faces of which are convex. In regard to radiation geometry, this provides that each beam which is incident through the entry end face into that cylinder lens inclinedly at a given angle relative to the axis of the system is refracted towards the axis of the system and thus towards the edge region of the exit end face which is also convex and shortly after issuing there passes through a focal point. A detector with an annular active zone is arranged at that focal point distance behind the rearward convex end face. Associated with a given spacing from the edge of the exit end face, corresponding to the diameter of the annular detector zone, is a given angle of incidence into the entry end face. That angle of incidence is correspondingly more greatly inclined with respect to the longitudinal axis of the system, for a given detector radius, and the sensor characteristic therefore has a correspondingly larger cone angle, the greater the refractive index of the material of the cylinder in relation to air. As is known, that index is about 1.5 in the case of optical glass, about 3.5 in the case of silicon and about 4.0 in the case of germanium. The detector therefore reacts to a hot spot passing into the sensor beam path which is in the shape of a hollow cone, with its cone spread angle being determined by the geometry and the material of the cylindrical lens.

[0007] Further features and advantages of the structure according to the invention and additional developments will be apparent from the further claims and from the description hereinafter of a preferred embodiment of the proximity sensor according to the invention, which is diagrammatically shown in the drawing in highly abstracted form, being limited to what is essential, and not being true to scale. The single FIGURE of the drawing is a view in axial longitudinal section through the beam path through positive lens of cylindrical shape, behind which the actual detector element is arranged in the image focus plane.

[0008] The thermooptical proximity sensor 11 diagrammatically shown in the drawing substantially comprises an annular detector element 12 which is mounted coaxially with respect to the axis 13 of the system behind a long cylindrical lens 14 of silicon or germanium. That cylindrical lens 14 can be considered as being cut out of a thick positive (that is to say convex at both sides) lens shape 15 in coaxial relationship with the axis 13 of the system. In the cylindrical lens 14 therefore disposed axially opposite to a convex entry end face 16 is an exit end face 17 which is also convex (but which does not necessarily involve the same curvature).

[0009] A parallel beam 18 which enters the convex entry end face 16 at a given inclination with respect to the axis 13 of the system is refracted by virtue of the beam-geometric aspects as diagrammatically illustrated towards the axis 13 of the system in the denser material and is concentrated onto a small exit region near the edge, which is dependent on the angle of incidence, in the exit end face 17 which is also convex. The focus is on the other side of the exit end face 17. Arranged at that focus is the detector element 12 disposed in a protective housing 19 behind a thermal window 20. As the detector element 12 is intended to detect only the edge region corresponding to a given angle of incidence, it is either in the form of an annular disc of suitable diameter and ring width; or a disc-shaped detector is covered over centrally so that only that ring zone is caused to respond. In this respect it is to be borne in mind that this plane-parallel disc of the window 20, due to the refractive action, on the way to the focus, results in further displacement of the beam path, as is known from geometrical optics as a refraction phenomenon when passing through a parallelepiped.

[0010] In order to be able to maintain the small distance for the focus behind the cylindrical lens 14 for positioning of the detector element 12, its convex exit end face 17 is ground flat in the centre transversely with respect to the axis 13 of the system in order here to fix the protective housing 19 with its window 20. The ground face is preferably matted or lacquered so that only the above-mentioned annular region of the detector element 12 can be subjected to the radiation effect. All that remains of the convex exit end face 17 therefore for the beam to pass therethrough is only a narrow concentric ring zone 22, along which the exit region 19 is displaced around the axis 13 of the system when the entry beam 18, while maintaining its inclination, is rotated about the axis 13 of the system, as is diagrammatically shown in the drawing for two entry angles which are emphasised in isolation. The sensitivity characteristic in the form of the wall of a hollow cone (represented by the beams 18), by way of the annular zone 22 at the exit end, therefore results in imaging of the individual incident rays on the annular detector element 12 which is disposed in the image focus plane of the beam path. That annular imaging effect involves an angle of incidence of the beams 18 with respect to the axis 13 of the system, which is correspondingly steeper, the greater the refractive index of the material constituting the cylindrical lens 14. Preferably therefore germanium or at any event silicon is used for that purpose.

[0011] As can be seen from the beam diagram and the detector element 12 which is responsive in an annular configuration, that proximity sensor 11 is insensitive ahead in the direction of its axis 13; it responds only when a heat-radiating object (here in particular the attacker which is to be defended against) passes into the inclined beam path in the shape of a hollow cone.

[0012] In accordance with the invention therefore there is provided a proximity sensor 11 for the radially operative warhead of a defence shell which is fired against an attacking projectile from the object to be protected in order to interfere with or even destroy the functionality of the attacker. That proximity sensor 11 does not yet respond to the attacker which is to be defended against by the shell appearing ahead in the direction of flight thereof; rather, the proximity sensor is to respond only when the attacker to be defended against is detected ahead inclinedly at an operatively optimal distance. Such a response characteristic in the form of the wall of a hollow cone is afforded if an annular detector element 12 is arranged in the image focus plane behind a positive cylindrical lens 14. 

1. A proximity sensor (11), for initiating the triggering of the warhead of a defensive shell directed against an approaching projectile, characterised in that said sensor has a response characteristic in the form of a hollow cone (beams 18).
 2. A proximity sensor according to claim 1 characterised in that it said sensor has an annularly responsive detector element (12) in an image focus plane behind a positive cylindrical lens (14).
 3. A proximity sensor according to claim 2 characterised in that the cylindrical lens (14) is a bar with convex entry and exit end faces (16, 17).
 4. A proximity sensor according to claim 2 or claim 3 characterised in that the cylindrical lens (14) comprises an infrared radiation-conducting material with a high refractive index in relation to air.
 5. A proximity sensor according to claim 2 characterised in that the detector element (12) is arranged in a protective housing (20) behind a window (21).
 6. A proximity sensor according to claim 5 characterised in that the detector element (12) is mounted behind a flattened centre, surrounded by a convex annular zone (22), of the exit end face (17).
 7. A proximity sensor according to claim 4, characterised in that said infrared radiation-conducting material is selected from the group of materials consisting of germanium and silicon. 